1. Field of the Invention
The present invention relates to organic optical components and preparation thereof. Optical components may be used to transmit and/or manipulate light signals in various fields of technology such as telecommunications, data communications, avionic communications, sensor networks, automotive control systems, display and backlighting.
More specifically components are intended for passive light transmission and/or manipulation including polarisation control, transmission, distribution, splitting, combining, multiplexing, phase filtration.
Active optical components requiring electrical connections to power and/or control circuitry are also envisaged.
Specific optical components of the invention include waveguides, strip waveguides, bends, Y-junctions, blazed grating couplers, power splitters, star couplers, holographic gratings, holograms, phase filters, circuitry interconnections, bridging devices, lens arrays and the like.
2. Description of Related Art
A diverse number of organic components are known. U.S. Pat. Nos. 4,593,974 and 4,547,040 (Yamamoto) disclose components comprising embedded organic optical fibres, obtained using extrusion techniques. U.S. Pat. No. 5,455,883 (Shigeta) discloses optical input connections for displays. Organic optical components comprising two or more polymers having different refractive indices are a means of providing more complex light guiding components with excellent optical properties. Increasingly these are favoured over the inorganic counterparts, the preparation of which is increasingly difficult in line with the need for need complex and miniaturised components.
Unfortunately organic optical components suffer high optical losses both by polymer absorption itself and by leakage from imprecise structures and misaligned components. Cross talk between components in high density systems is also a problem.
Organic optical components usually comprise two or more polymers having different refractive indices. The polymer having the higher refractive index when surrounded by the other polymer can function as a waveguide which allows for the transmission and manipulation of a light signal. The higher refractive index polymer is usually introduced into the other resin which is then cured.
Specifically polymer moulding processes have been used involving the filling of mould cavities to produce rigid mouldings with flat faces which can be further processed to provide polymer analogues of optical devices produced using inorganic materials, for example as disclosed in U.S. Pat. No. 5,136,678 (Yoshimura) and U.S. Pat. No. 5,113,471 (Inaishi). Two flat surfaces one provided with channels for filling with resin when the two surfaces are squeezed together is an unreliable method of producing well defined channel waveguides due to the difficulty of avoiding a thick layer of material being formed between waveguides than can allow optical cross-talk between channels. Bonding the two surfaces together before introducing the channel filling resin can be achieved by capillary filling but is time consuming and can be unreliable for complex and large area. Yoshimura and Inaishi disclose a method for preparing optical waveguides by first injection moulding a flat substrate with a number of grooves, next an UV curable resin is filled in the grooves to form a core material. The resin is then irradiated with UV light to cure the core material. Finally a coating is applied to the substrate and core material by a spin, spray, casting, etc method. This procedure an uneven residual layer since a portion of the resin inevitably spills outside the grooves. As the guides described by Yoshimura and Inaishi are very simple, ie no bends or junctions which are overlay critical, it is likely that the importance of an overlay was not appreciated by Yoshimura.
More recent investigators have found that trying to fill injection moulded waveguides by squeezing out the excess (as in Yoshimura) is not a reliable method and have resorted to filling waveguides that have enclosed channels performed using a top cladding by capillary filling or using vacuum assistance, ie using liquid crystal cell filling methods.
Prior art batch processes have been used to define channel waveguides in inorganic or polymer coatings on rigid wafers using lithographic, multistep, materials addition and subtraction processes.
Other production techniques have been developed specifically to avoid spillage of resin or as an alternative dispersing with the need to inject resin, in an attempt to attain acceptable optical losses, uniform output and consistent performance, increasing the incentive to use optical components. However with these there is a practical limit to the size of waveguides which can be manufactured. Since the resin must be poured in grooves in a flat substrate, the groove must be sufficiently large to receive the resin, and the device for pouring the resin must be sufficiently sophisticated to ensure the resin does not overflow the grooves. Therefore prior to the invention, polymeric optical waveguides smaller than 100 micron were not manufactured which did not have a relatively thick layer of polymer of variable thickness which produced unacceptably high losses.
Lithographic techniques, for example, as disclosed in Inamura et al, xe2x80x9cElectronics Lettersxe2x80x9d, 27 (15), 1342-1343 (Jul. 18, 1991) and Matsura et al xe2x80x9cElectronics Lettersxe2x80x9d, 29 (3), 267-271 (Feb. 4, 1993) conventionally entail fabricating a waveguide comprising of core and buff layers on substrate by waveguide patterns fabricated using photolithography, core ridges then formed by reactive ion etching until the buffer layer surface is exposed and finally the core ridges covered with a spin coated cladding layer, relies on immersing the waveguide pattern entirely subsequent removal of residual material. U.S. Pat. No. 5,265,185 discloses a similar process using a combination of spin coating and then etching. Although the resulting product has no residual overlayer as a result of the reactive ion etching, the technique is time consuming and laborious and not suited to further miniaturisation. Imamura discloses mono and multi mode waveguides comprising film thicknesses of 8 and 15 micron using specific low optical loss polymers and reporting low optical losses.
Capillary techniques, for example as disclosed in U.S. Pat. No. 5,265,184 (Lebby et al) are also known as a means to avoid residual overlay from ation. In these techniques a waveguide mould is obtained and an encasing lid glued on top. Waveguide shaped cavities are then filled by capillary action with resin subsequently curing. Lebby injects resin into the channels. It is important to avoid cavitation, bubble formation, striations and the like. These considerations become more important the more complex the optical component, the finer the channel and the greater the total mould surface area. Accordingly complex or extensive products obtained using this technique are commercially unviable.
A further technique which avoids overlay formation is known as selective photopolymerisation, for example as disclose in EP 0 420 592 (Nippon Telegraph and Telephone Corporation), U.S. Pat. No. 5,265,185 (United States of America, Secretary of the Army) and Booth et al J Heatwave Technol, 7(10), 1452. Monomers contained in a polymer are selectively polymerised to change the refractive index to make a pattern like optical waveguide. Specifically a mask having a predetermined patter is mounted on a polymer sheet or substrate composed of a transparent polymer which contains a low refractive index monomer and the sheets or substrate are irradiated with UV rays through the mask to selectively polymerise the low refractive index monomer, replicating the patterns. The photopolymerised portion of the polymer has a lower refractive index than the polymer matrix. The polymer sheet is heated in vacuum toremove unreacted monomers which remain in areas unexposed by UV rays. As a result unexposed portions of the polymer consists of the high refractive index polymer alone. The patterned sheet may then be sandwiched by cladding layers of low refractive index polymer to produce an optical component. Variations such as selective cross linking and are also disclosed. This technique suffers from variations in refractive index depending on residual monomer content, and protrusions and depressions formed during development of the polymer, further distorting the optical properties. Channel base and side wall roughness also becomes a limiting factor in this technique as does cross talk between channels. Further examples of this technique are disclosed in Kurokawa et al IOOC 1997, A8-3, 121 and AO 19 (18) Sep. 15, 1980, 3124-9, Hartman, Topical Meeting on Integrated and guided wave optics 1988, 62-5, Truhalla, SPIE, 1177, Integrated optics and optoelectronics (1989) 379-386 and OFC 1991, Feb. 19, 1991 and Izawa et al, European Conference on Optical Communications, ECTS 1991, 97-100.
Finally, a third technique which avoids problems associated with the foregoing methods involves moulding and embossing, for example as disclosed in U.S. Pat. No. 5,171,293 (Iida). A mould is fabricated to include a waveguide relief. The mould is used to emboss the waveguide or grating relied into a thermoplastic polymer structure which is then cured. The structure is then filled with higher index polymer and the structure encased thereby squeezing the polymer filling. The enclosed polymer is then cured. The top cladding may be retained or replaced in the final optical component formation. Examples of this technique are disclosed in Dannberg et al and Neyer et al, European Conference of Optical Communication, Montreux 1993, Proc 437-440 and 337-340,, Pahlke et al, Tech Messen 60 (93), 339, Baraldi et al, Proceedings of the 18th Conference on Optical Communications (ECOC) 1992, 56-64 and Rogner et al, SPIE Vol 1506 Micro-Optics II (1991), 81-91. In this case it is difficult to avoid formation of an overlay moreover the overlay thickness is highly prone to variation since the top cladding surface is unlikely to be planar, nor is the open top of the embossed thermoplastics substrate and this will lead to unevenness. The difficulties are moreover likely to be greater the greater the size of the structure. The overlay thickness is also likely to be significant since it is difficult to expel excess material simply by pressing a plate onto the embossed thermoplastic substrate.
Accordingly there is a need for organic optical components which do not suffer from the inherent fabrication distortions described above, specifically do not comprise a thick and uneven residual layer between channels or if having no layer at all between channels do not suffer from poor optical qualities within the channels.
Moreover there is a need for a greater precision in dimensioning of organic optical components with a view to the wavelength of light to be transmitted. Specifically a component having particular dimensions may operate in multimode for a given wavelength of light, such as visible light, whilst performing as mono mode for a different wavelength such as ir. By tailoring the component dimensions specifically to an envisaged wavelength of radiation the optical losses may be dramatically reduced. This necessitate however a very high accuracy in both the wavelength cladding and core.
Moreover there is a need for highly complex and high density optical components comprising mono and multi layer structures with precide interconnection of components to avoid spillage for example by mismatching a waveguide to an optical fibre and the like. Processed which provide an optical system in a single production cycle allow precise matching an control of components thereby dramatically reducing optical losses. Accordingly there is a need for an integrated optical system comprising integrated components.
We have now surprisingly found an organic optical component and preparation method therefore which provide controlled introduction of resin as a waveguide in manner to control overlay formation, and which overcomes the above disadvantages. The component provides a number of additional advantages in terms of performance, processing and preparation of complex systems.
In its broadest aspect there is provided according to the present invention a polymer thin film comprising at least one polymeric relief structure for use as an organic optical component, comprising
a) a first layer of a first polymer;
b) an optically transmissive UV curable second polymer having at least one relief feature, the second polymer having a refractive index which is suitable for transmission or manipulation of light; characterised by
c) an overlay of the second polymer about the at least one relief feature having a thickness in the range 0.001 micron to 1.5 micron over the surface of the first layer.
The second polymer layer may be supported on the first layer and protrude therefrom or may be contained as a core within the first layer as a cladding layer. In this case the second case projects inwardly into the first layer, and in the case that the second layer comprises a plurality of relief features, there are a plurality of projections whereby the two layers are effectively co-continuous.
In a first aspect there is provided according to the present invention a polymer thin film comprising at least one organic optical component for use as an organic optical component which polymer film comprises
(a) a first layer of an optically transmissive first polymer having a first refractive index, the first layer having a surface in which is defined at least one retaining feature adapted to retain a polymer therein;
(b) a retention layer of an optically transmissive UV curable second polymer retained within the at least one retaining feature, the second polymer having a second refractive index which is greater than the first refractive index; characterised by
(c) an overlay of the second polymer over the first layer adjacent the at least one retaining feature, the overlay having thickness in the range 0.001 xcexcm to 1.5 xcexcm over the surface of the first layer.
1. Preferably the polymer thin film comprises additionally
(d) a second layer of an optically transmissive third polymer having a first refractive index which is less than the second refractive index and which second layer contacts the second polymer retained within the at least one retaining feature and the surface of the first layer
wherein the organic optical component exhibits an optical loss of less than 2.0 dB cmxe2x88x921 at at least one wavelength in the range 300 to 1600 nm.
The components of the invention provide the following advantages:
1 Photocurable polymer formulations that provide the range of optical properties required particularly refractive index and effective transmission at the desired operation wavelength(s).
2 Precision moulded channels or retaining features for forming optical waveguides and support structures for other components that require precise alignment with the waveguides.
3 Light guiding cores with insufficient overlayer of material preventing optical leakage to adjacent waveguides enabling low noise optical signals processing within high densities of guides using optical circuitry functions such as splitting, combining, multiplexing, demultiplexing.
4 Flexible waveguides covering large areas sufficient for use with, for example, displays and in continuous lengths such that the product forms a conveyor belt for ease of component integration and mass manufacture. Simple cutting, slitting, encapsulation processes can be used for final product conversion.
5 Functional layers can be added to the back or front of the product by additional lamination, film substitution, coating and/or embossing.
6 Pick and place technology can be used to insert components on the conveyor belt assembly line.
The thin film of the invention may be provided as a composite with additional layers of polymer or other support materials as hereinbefore defined or may be unsupported.
The thin film has any length and width dimensions that facilitate winding on to a reel.
The film may comprise any one or more optical structures such as waveguides and the like which may be useful as an integrated system or which may be cut into portions as individual optical components.
The overlay of the second polymer has a maximum thickness of 1.5 micron. The thickness of the layer may be selected in the range 0.001 to 1.5 micron, whereby a lower value is selected when it is desired to transmit light of shorter wave lengths and any value in the range may be selected when it is desired to transmit longer wavelength light. It will be appreciated that the overlayer thickness should not exceed the selected value however it may be less than that value or may even be 0 thickness in regions. Accordingly the layer thickness may vary between any desired value in the hereinbefore defined range and 0. In some instances it may be preferable to attain a high level of uniformity in which case a suitable variation may be in the range +/xe2x88x920.75 micron (provided that that value does not exceed a total of 1.5 micron or is not less than 0), more preferably +/xe2x88x920.25 micron (within the same constraints), most preferably +/xe2x88x920.1 micron (subject to the same constraints).
Preferably the film is in a form of an elongate film of length in excess of 2 m, or a portion thereof.
The polymeric film may be useful for providing optical components for mono or multi mode operation, by selection of overlay thickness and wave length of light to be transmitted.
The polymeric film preferably comprises optical structures comprising integral positioning aids for positioning in optical communication with further optical components. These may be provided at any one or more of the edges of the film or structure or may be provided at the edge of each component provided on the film. It is a particular advantage that the positioning aid is integral with the component. This avoids undue losses which may be caused by imprecise positioning.
Preferably a portion of a polymeric film as hereinbefore defined comprises at least one laser cut edge. This provides precise dimensioning of the component which is thereby portioned off and moreover provides a totally smooth incision which is suite for precise connection within an optical system avoiding undue losses.
The polymer film may comprise any additional supporting or other functional layers onto which it is suitably laminated by self adhesion or with use of additional adhesive, preferably an optical adhesive.
The thin polymer film may comprise optical components having any desire dimensions. A plurality of optical components on a single film may be the same or different both in terms of configuration and dimension. The film is particularly suited for providing components having at least one dimension selected from height and/or width and/or diameter in the range 1:250 micron, preferably 5:50 micron or 1:10 micron, more preferably 5:10 micron or 1:5 micron
The supporting substrate material and thickness is preferably chosen so that the laminate remains flexible. This has the advantage that the method of preparation can be run as a continuous reel to reel process. The product from this process comprises a reel of the film laminate. That laminate may be subsequently slit or cut to separate out individual smaller components or the product may largely comprise the uncut film laminate thereof.
The first layer may be supported by a suitable substrate which may be subsequently removed from the first layer. Alternatively, the substrate may remain with or is subsequently applied to the first layer thereby providing a laminate structure in which the relatively expensive first layer is of a thickness which is sufficient to provide the desired depth of retaining feature and complementary optical contrast and the substrate provides the desired level of support and may also provide additional positioning features for such items as optical fibres and other optical components. Thus in a preferred embodiment, the first layer is supported by a thicker support layer which possesses positioning grooves which are in alignment with the at least one retaining feature and which grooves are able to receive and locate optical fibres in abutment to the end of each at least one retaining feature. It is particularly preferred that the optical fibres are so located during the application of the second polymer and that sufficient of the second polymer is applied such that a suitable optically correct joint is formed between the end of the retaining feature and the abutting end of the optical fibre.
The second layer may be provided by coating the third polymer onto the first layer or precoating the optionally releasable dispensing layer,. Conveniently, the dispensing layer provides the second layer such that the second layer is superimposed upon the surface as a consequence of the advancement of the line of contact. The second layer may also be provided with retaining features in which is retained an optically transmissive polymer, which may be the same as the second polymer and which may be so placed that at least some of the retaining features of the first layer are matched with at least some of the retaining features of the second layer such that they can form a composite optical component.
The optically transmissive polymers may be selected from those known in the art including those developed as light curable adhesives for joining optical components for example those sold under the LUXTRAK (LUXTRAK is a trade name of ICI plc), those developed for polymer optical fibre fabrication and those developed for optical recording using polymer photoresists.
In particular the optically transmissive second polymer may be formed from a suitable resin for example halogenated and deuterated siloxanes, acrylates and methacrylates such as ethyleneglycol dimethacrylate, tetrafluoropropylmethacrylate, pentafluorophenylmethacrylate, tetrachloroethylacrylate, multifunctional derivatives of triazine and phosphazene. Resins and polymers that contain highly fluorinated aliphatic and aromatic moieties, particularly those capable of being formed into cross-linked or covalent networks, are preferred. Additionally the second polymer may comprise a resin or polymer formulation that provides a non-linear optical (NLO) property such as described in European patent application EP-0474402-A, second order NLO properties, third order NLO properties or signal amplification properties.
The optically transmissive first polymer in which the retaining feature is formed may be prepared from curable resins or can be fabricated from substituted and unsubstituted polymers selected from polymethylmethacrylate, polycarbonate, polystyrene and polyimides.
Suitably for the preparation of monomodal organic optical components, in particular those in which the retaining features have a height and a width from 5 to 10 xcexcm, the difference between the second refractive index and the first refractive index is from 0.001 and 0.02, preferably from 0.002 and 0.01 and particularly from 0.004 and 0.007, i.e. the difference is from 0.05 to 1.5% of the first refractive index. Where the retaining features have a height and a width from 1 to 5 xcexcm and in which instance the reduction in the thickness of the overlay is even more advantageous, it is preferred that the difference in refractive index is from 1 to 30%, particularly 1 to 20%, and especially 1 to 5% of the first refractive index.
Preferably, the second polymer has a refractive index from 1.4 and 1.7, for example from 1.43 and 1.57.,.
Where the second polymer is coupled to the end of an optical fibre it is preferred that the second polymer has a refractive index which closely matches that of the optical fibre. In such circumstances it is also preferred to select a second polymer which shows good intrinsic adhesion to the optical fibre which is principally formed from silica.
The refractive index of the polymers may be modified by the inclusion of suitable additives to the polymer, in particular mixtures of those polymers mentioned above may be used to achieve a desired refractive index. Where the major component of the second polymer is bis(oxy ethyl methacrylate) 2,2xe2x80x2, 3,3xe2x80x2, 4,4xe2x80x2 hexafluoroglutarate, it is particularly preferred that the refractive index of the second polymer is adjusted by adding appropriate amounts of ethylene glycol dimethacrylate which can increase the refractive index (as measured at 1.32 or 1.55 xcexcm) by an absolute value in excess of 0.02 when added at a level of 30% by weight.
The overlay of the second polymer has a maximum thickness of less than 1.5 xcexcm, preferably less than 1 xcexcm, and particularly less than 0.5 xcexcm over the surface of the first layer. The average thickness of the overlay is preferably less than 1 xcexcm and particularly less than 0.5 xcexcm. The variation of the thickness of the overlay across the surface is preferably less than xc2x10.75 xcexcm, particularly less than xc2x10.5 xcexcm and especially less than xc2x10.25 xcexcm. When formed into an organic optical component the component will exhibit an optical loss of less than 2.0 dB.cmxe2x88x921, preferably less than 1.0 dB.cmxe2x88x921 and particularly less than 0.5 dB.cmxe2x88x921.
The at least one retaining feature is conveniently in the form of a channel, and typically is of a generally trapezoidal cross-section although other configurations are possible, for example rectangular, square or semi-circular. As stated above, for a monomodal organic optical device, the height and width of a retaining feature may be in the range 1 to 10 xcexcm depending on the difference in refractive index used. However, preferably the height and width of the retaining features are in the range of 5 to 10 xcexcm, and particularly in the range 6 to 8 xcexcm. Preferably the maximum width of the retaining feature is at the surface of the first layer. Typically, the ratio of the maximum width to minimum width is less than 2:1 and is preferably about 1:1. The retaining feature may be formed by any suitable technique for example a lithographic etching process such as reactive ion etching through a suitable mask although moulding or embossing is preferred. Where the organic optical device is to be used as a multimode device then similar fabrication techniques can be used with suitable adaption of the dimensions and selection of materials, usually though the height and width of the retaining features when employed in a multimodal device are in the range 10 to 100 xcexcm, for example 10 to 50 xcexcm.
In order to facilitate the curing of the resin it is preferred to use an initiator, for example a thermal and/or photoinitiator and particularly an initiator which does not absorb light at the operating wave length of the organic optical component. Typically, when used, an initiator is present in the resin at a concentration from 0.1 to 3.0% by weight, and preferably from 0.5 to 2.0% by weight. Suitable photoinitiators include 2-methyl-1-[4-(methylthio)phenyl)-2-morpholino propanone-1 (Irgacure 907), 1-hydroxycyclohexyl-phenyl ketone (Irgacure 184), isopropylthioxanthone (Quantacure ITX), Camphorquinone/dimethylaminoethylmethacrylate.
The method of manufacturing a waveguide film laminate where the waveguides are continuous along the length of the film would be as follows;
(a) an embossing roller is provided with a raised surface relief of the dimensions required to form the at least one retaining feature. The surface relief runs around the circumference of the roller in a continuous manner.
(b) A polymeric structure for use in the preparation of the continuous waveguide laminate is made by coating one side of the flexible substrate with a first polymer layer and contacting this layer with the embossing roller so as to transfer the retaining structure of the waveguide into the first polymer layer. The first polymer layer is cured while in contact with the embossing roller and then the film released.
(c) A line of contact is formed between a flexible dispensing layer and the surface of the first polymer layer and the surface is progressively contacted with the flexible dispensing layer such that the line of contact advances across the surface.
(d) Sufficient resin is applied, capable of being cured to form the second polymer and to substantially fill the waveguide features, along the line of contact.
(e) sufficient pressure is applied along the line of contact such that
(1) substantially all of the resin which is surplus to that required to fill the waveguide features at the line of contact progresses with the advancing line of contact thereby filling the waveguide features with resin; and
(2) no more than a quantity of resin capable of forming the overlay of the second polymer passes the line of contact; and
(3) the resin filling is cured in the waveguide features.xe2x80x9d
The supporting substrate material may be subsequently removed from the first layer.
Alternatively, the substrate may remain with the first layer providing a flexible laminate structure in which the relatively expensive first layer is of a thickness which is sufficient.
As the line of contact moves across the surface of the first layer the resin is effectively pushed across the surface and flows into the at least one retaining feature. The rate at which the line of contact advances across the surface will depend, amongst other things, on the characteristics of the resin. Typically, the resin has a viscosity from 0.1 to 100 poise and more typically from 10 to 100 poise. Preferably the line of contact is moved along the length of the retaining feature, particularly when the retaining feature had a width or height greater than 10 xcexcm, thereby improving the filling of the retaining feature and reducing the possibility of inclusion of gas bubbles.
The resin may be fully retained with a retaining feature as the line of contact moves from the retaining feature, in which case the resin may be cured at any convenient subsequent time. However, the resin may often show some degree of resilience in the non-cured form in which case as the line of contact moves from the retaining feature the resin therein will tend to relax and stand proud of the retaining feature thereby reducing the effectiveness of any optical component subsequently formed from the polymeric structure. To counter the relaxation of the resin it is preferred that the resin is cured whilst the line of contact passes over it. Conveniently and preferably therefore, the resin contains a photoinitiator which is activated by a particular wavelength of light, particularly UV light. A suitable source of light may then be used to cure the resin before the pressure applied along the line of contact is released and before the resin relaxes from the retaining feature. It is especially preferred that the dispensing layer is transparent to the light used and that the light is alone through the dispensing layer towards the resin. In order to focus the light substantially at the tip and thereby avoiding, for example, premature curing of the resin, the angle of incidence of the light onto the line of contact may be required to be adjusted from polymer to polymer. Alternatively, for a given angle of incidence and where the first layer is at least partially transmissive to the light, the first layer may be chosen to have a thickness such that the internal refraction of the incident light acts to focus the light at the line of contact. Additionally, where the first layer is at least partially transmissive to the light and is of a suitable thickness, a mirrored support may be positioned under the first layer thereby causing the transmitted light to be reflected back to the line of contact.
The pressure is applied along the line of contact by any suitable means. Suitably, the pressure is applied using a roller under a compressive load which may thus on rotation retain the resin in the nip formed by the roller between the buffer layer and the surface. It is therefore preferred that the resin is cured at the nip as the line of contact progresses across the surface.